Recently, attention in global change science has shifted away from predictions of trends that imply linear or steady rates of change in, for example, temperature and precipitation, weather events such as storms, heatwaves and cyclones, and responses to these phenomena in human and natural systems. The possibility of significant, non-linear or abrupt change is now increasingly recognized; that is, human-induced or -exacerbated environmental change may be characterized not by smooth transitions but by surprise, threshold effects and abrupt shifts.

This paper summarises recent scientific work on abrupt change, with selected examples of apparent sudden shifts in environmental variables. These phenomena define a particularly difficult set of research and policy problems within the sustainability domain. However, the magnitude and direction of such changes are attended by pervasive uncertainty, and provide both an opportunity and a deep challenge to research and policy. The opportunity is to allow a focus on the historical and current capacities of natural and human systems to cope with past or existing patterns of abrupt change, providing considerable data and experience to be drawn upon. The challenge is that radical uncertainty is not well constructed or handled by either traditional disciplines, and especially by modern policy systems. Most attention in global change in Australia has been focused on agriculture, selected public health issues, and to a lesser extent on weather events and human settlements, with a relative lesser focus on whole urban systems. This paper proposes some implications of abrupt change for urban Australia, and offers an initial scoping of research and policy challenges, focusing on increasing understanding of the parameters of possible non-linear change and its impacts, the capacity of policy and institutional systems to respond, and implications for styles of governance.